In my opinion 4 1/2 years old is young to be jumping full courses and asking for a lot out of her. She is still growing and her body is still maturing. I would have a chiropractor out for her and adjust her; I would also spend time, when she is able, doing exercises that build and stretch her muscles. Long trotting, hill climbing, backing up and down hills, flexing and stretching on the ground. Talk with the farrier about her break over, or maybe pain is causing her to land wrong and she is dragging her toe rather then landing flat. It could be a number of things get the chiro out and see if that helps, then the next step would be the vet.